Are you considering a career in PLC programming? Awesome! Learning how to program PLC’s (and how to debug under pressure) is an incredibly fun and rewarding experience. Maybe you’ve been considering attending a tech school, or have been talking to a friend who is a licensed electrician or who works for an automation integrator. Maybe you’ve developed an interest in industrial automation on your own and want to know more about becoming a controls engineer or PLC programmer.
Where are you in your journey? We want to hear from you in the comments!
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Whatever your background and level of interest, here are five suggestions for what might make PLC programming the right job for you!
1. You Think Automation Is Cool
But, I mean, who doesn’t, right? If the thought of setting up new systems, developing algorithms and sequences of operation, and programming robots and Programmable Logic Controllers sounds like it’s up your alley, then why not go for it?
Not everyone gets to do what they love, but if you have a true interest in automation, becoming a PLC programmer will allow you to learn how to establish and control automated processes. What could be cooler than that?!
2. You Love To Learn
Being a good programmer means treating every situation as an opportunity to learn more about the PLC, the programming template, the production line, and the equipment you’re controlling. As the “controller of the controller,” if you will, it’s not enough to be knowledgeable about just programming. The whole point of the PLC is to accept inputs and set outputs to and from alllllll the many field devices – conveyors, robots, I/O blocks, fixtures, actuators, sensors, servos, … , the list goes on forever.
Industrial automation is home to a thriving ecosystem of devices, equipment, and communications protocols. For instance, every imaginable flavor of communication can be found out there, from modern 10/100 (and faster) Ethernet to 9600 baud serial connections. As someone who designs and debugs PLC logic, you will need to work to grow your understanding at every layer of your production equipment.
With so many different systems to learn, you must strive to build your knowledge of everything in the plant. From physical cabling to vendor-specific camera solutions used on your vision systems, eventually, it’s going to break! When it does, people will be looking to you to come up with solutions to highly technical issues.
Long story short, as an industrial electrician, electrical engineer, or anyone else who does PLC programming, you should be learning something new every day. Don’t worry – what you don’t learn on your own, the breakdowns will teach you. 😀
3. You Love Solving Puzzles
Often times, as an automation engineer, you will find recurring issues with the equipment that no one knows how to fix. As such, it will be your job to figure out what’s wrong.
… a bug in the original programming is allowing 1 in every 100 units to go through without a certain part being installed
… ten times a day a certain fault occurs and stops production, and there doesn’t seem to be any discernible pattern as to what’s causing the issue
… you’re having trouble with a complicated sequence of events that occurs within milliseconds
… you have to debug an exchange of data between two PLC’s that takes place over your industrial network
As a PLC or robotics programmer or engineer, people will often look to you to “deep dive” the tough, troublesome, technical issues that evade understanding. When you are working in that role, it’s likely that you will be presented with difficult troubleshooting situations that will require a deep understanding of the equipment and technology in question.
4. You Work Well In Challenging Situations
The first rule of PLC programming is: all logic requires debug. Whether you’re setting up a new process or troubleshooting an issue with an existing system, as a “controls guy” (or “controls gal” 🙂 ), people will be looking to you to get the equipment working again.
You know what they say – no guts, no glory. When the equipment isn’t working, excellent programmers or controls engineers are able to enter challenging situations with a cool head. Once you’ve quickly gained an understanding of the problem, it’s time to work with your teammates to determine and correct the issue.
Adept PLC programmers who work in production (manufacturing) environments will likely be called to the toughest breakdowns. As your skills and knowledge grow, you will become more and more comfortable working in tense, time-sensitive situations. Wherever you are in your journey, the key is to always strive to learn more.
5. You Love To Think Creatively, Logically, And Analytically
Lastly, PLC programmers need to be able to:
Analyze a situation or a problem to be solved
Think of a possible solution
Come up with a way to turn that solution into logic
Then, write that logic and debug, debug, debug!
When you’re striving to make equipment run, it’s sometimes easy to get tunnel vision and hone in on one possible solution. As such, PLC programmers need to be aware of the logic and the equipment with which they’re working, so they can pull the right programming “tool” out of their “tool bag” and put it to work to solve the problem. Sometimes, you have to step back and think, “is this the only way I could attack this issue, or could there be an easier way?” This may be the best way to jar yourself out of the tunnel and start thinking creatively.
What Do You Think?
Do you fit the bill? Tell us what you think in the comments below! We want to know where you’re at in your own journey towards becoming a world-class PLC programmer.
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If you’re considering a career in Industrial Automation, you are probably starting to hear a lot about PLC’s. If you haven’t worked with PLC’s before, they might seem a bit mysterious; you might be wondering what a PLC is, exactly. Let’s shed some light on the topic for you below.
What exactly is a PLC? A PLC, or Programmable Logic Controller, is a computer that is used for processing inputs and setting outputs. When I say “computer” in this context, I don’t mean what we typically think of when someone says “computer” – although a PC can function as a PLC with the right software.
A PLC is (typically) a computer in the more general sense – it is a microprocessor that is connected either directly or via an industrial network to some inputs and outputs, and it is programmed to evaluate the inputs and set the outputs in such a way that it facilitates the execution of some process. In other words, it is a processor, similar to the one in your computer, that adheres on a very basic level to the “IPO Model“: it accepts inputs, performs processing, and sets outputs.
Try to think of it this way – a factory is, in a sense, a giant machine that takes in a lot of parts and turns those parts into some product or family of products. Within the factory, there are some number of automated production lines, with each line performing some process on the parts.
On some lines, robots may move parts around so that other robots can weld them together. Some lines may have stations where operators use special equipment to install fasteners in each part before it moves to the next station. Some lines may simply convey parts to another area of the facility. In any of these cases, think of individual automation lines as large machines themselves; they perform some custom function within the factory to contribute to the assembly of the final product. Generally, they turn some combination of parts into a larger sub-assembly.
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PLC’s are the brains of these machines, much like your car’s ECM is its brain. In fact, in many regards, an ECM is to a car as a PLC is to an automation line in a factory – an ECM is, effectively, a PLC for your car.
An ECM accepts inputs from the vehicle, such as:
Oxygen sensor feedback
The ECM then performs calculations using the input data, to provide appropriate outputs:
Injector duty cycle
I guess the list above is most relatable if you have a bit of automotive knowledge. Here’s another comparison that might be more broadly intuitive:
The home automation hub processes those inputs in accordance with how you have it set up, and then provides appropriate outputs:
Turning lights on and off
Closing the garage door
While both comparisons are reasonable, there is one manner in which both examples are significantly different from PLC’s:
ECM’s and home automation hubs come pre-programmed from the manufacturer. Their programming is pre-determined and, while there may be features that you can set up or configure at the user level, the code that the processors are executing is generally not editable.
PLC’s, on the other hand, are blank slates – they are putty in your hands, so to speak, for you to program however you need, in order to control your automated processes.
What do you need to have to be able to program a PLC?
In order for a PLC to do the incredible job of controlling equipment, you must provide it with the programming instructions, or “logic”, that brings your process to life. As a broad statement, you will need the following to connect to and program a PLC:
The PLC, with its power supply and any necessary accessories
A PC or programming terminal with the PLC vendor’s software
A means of communication between the terminal and PLC
Varieties of PLC’s
PLC’s come in all shapes and flavors, from this relatively affordable Electrodepot PLC to this Siemens PLC that can cost a few thousand dollars – which is the refurbished price, by the way. This Siemens NCU is just one example of a high-end PLC – as far as pricing for the cream of the crop, a few thousand isn’t even the upper bound.
There are many, many categories in which PLC’s can differ, but here are some noteworthy parameters:
Physical type (stand-alone, modular, or rack-mounted)
What are the PLC’s capabilities for expansion?
What types of accessories are available in this PLC’s product family?
Allen Bradley/Rockwell Automation PLC’s, on the other hand, require licensing to use their software – to the tune of $1,000’s for an annual license
Communicating with the PLC
Just as there are many different types of PLC’s, communicating with the PLC can mean many different things depending on what PLC you’re using:
Some PLC’s have programming ports that you can hook up to via a USB cable
Some require proprietary (read: expensive) programming cords that are vendor-specific
Many modern PLC’s can be programmed via an industrial network; like your router at home, you can connect to the PLC over Ethernet cabling.
What language is used for PLC programming?
Trick question? There are multiple languages that are used for PLC programming, but the most common is “ladder logic”, the technical nomenclature for which is actually “Ladder Diagram”. Two other common PLC programming methods are Function Block (or Function Block Diagram) and Structured Text.
Shown in the image above), ladder logic is the most common PLC programming method. Ladder logic is highly visual, and shows the truth of each instruction via visual indicators on the programming interface. Once you learn the meaning of a small group of instructions, you’ll be able to start reading and understanding simple bits of ladder logic.
The “ladder” in ladder logic comes from the appearance of the visual interface. Reminiscent of relay systems, ladder logic shows “energized” vertical bars on the left and right side of the screen, with each line of logic represented as a literal horizontal line – or “rung” – extending between the two vertical bars. Multiple “rungs” on the screen are visually similar to a ladder, hence the name.
Despite the high-level, visually simple nature of ladder logic, modern interfaces are extremely powerful and permit essentially any function of the PLC to be leveraged.
Function Block Diagram
Function Block Diagram, or Function Block, is another visually representative programming interface. With function block, instructions are laid out like landmarks on a map, with inputs and outputs from each instruction branching out like roads to connect to other instructions. Function block can help to visualize a process and its inputs and outputs.
Many of the basic building blocks of ladder logic are available when using function block. These basic functions include comparative operators (equal to, greater than, etc.), counters (increment or decrement a count whenever X occurs), timers, and many more.
Like many ladder logic interfaces, you can develop your own instructions in function block. Creating custom functions allows you to reuse logic when applicable to multiple devices or programming contexts.
A third method of PLC programming is Structured Text. For those familiar with computer programming, structured text will seem very familiar. Similar to high-level programming languages such as BASIC, structured text allows the user to write code to control the PLC.
In the PLC world, it’s important to consider how much trouble it will be for other people to deal with your logic when something isn’t working. Because structured text is less visual than either ladder logic or function block, it may not be as readily accessible to technicians who don’t have experience with the type of computer programming of which structured text is reminiscent.
No logic is immune from future debug when the equipment fails. As such, you want your PLC programming to be easy for others to read and understand. For this reason, I prefer a more visual interface than that offered by structured text, if only to make my logic accessible to a larger portion of the people who might have to debug it.
How can I learn more about PLC’s?
Three ideas come to mind for how you can start learning more about PLC programming:
Attend formal training for PLC programming
Colleges offer degree programs such as Industrial Automation and Mechatronics that provide formal training in PLC programming
Certain skilled trades apprenticeships (including many electrician apprenticeships) will also require formal study of PLC programming
Find PLC programmers in your network, on LinkedIn, or in other forums who may be willing to tell you a bit about the industry and what to expect
Buy your own PLC and start experimenting to build your skills
Get yourself an inexpensive Arduino starter kit and start building projects that will help you understand the Input, Process, Output Model
Arduino is not technically considered a PLC, but, basically, it’s an entry-level PLC
One drawback with Arduino is that you won’t be using ladder logic; it’s programmed with C
Many of the other experiences with Arduino will be identical or directly analogous to PLC programming
You will set up inputs such as sensors and switches
You will write logic that will handle or take action based on the inputs
Your logic will set outputs to turn on lights, activate motors, etc.
Another nice thing about this kit is that it comes with a slew of sensors, lights, resistors, etc., and a project guide
If you have the money, fork over the dough for something like the Electro Depot or EZRack PLC starter kits
These kits will provide a very realistic experience, using their vendor-specific software to program in ladder logic
Both of these kits have simulation functions, allowing you to experiment with your logic without needing a ton of extra equipment
Tell Us About It
We hope the information above will help you in understanding some of the basics regarding PLC’s. Where are you at in your own journey? Are you considering a career in industrial automation or specifically as a PLC programmer? Let us know in the comments.
If you’ve become involved in the world of industrial automation, you may have heard the terms “integrator,” “automation integrator,” or “automated systems integrator.” The term “systems integrator” exists in both the fields of industrial automation and information systems. There are some blurred lines between those fields, as both involve networking, computers, and programming. In this article, we’ll go over the concept of automated systems integration within the fields of industrial automation and manufacturing.
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Let’s say you’ve just opened a new business. You want to manufacture pencils.
Your company has a solid business plan, funding, a building, your management team, and employees. You know the colors that you want to manufacture and even how you want to market your products. There’s just one little problem… You don’t know anything about how to actually manufacture pencils. That is where a systems integrator comes in.
An integrator is a company that specializes in bringing systems, equipment, and machinery together to create a manufacturing solution. Thus, an integrator “integrates” whatever types of equipment and controls are required to turn separate machines into an assembly line (or other automated solution). The machines that are integrated may or may not be designed to work together easily. To do the job, the integrator must overcome any technical challenges to build a complete solution.
In this sense, an integrator transforms unrelated machinery and electronics into a factory. You can think of a factory as a sort of monstrous Rube Goldberg machine that converts raw materials into marketable products.
What Kinds of Equipment Does An Automated Systems Integrator Have to Bring Together?
Consider everything going on in the image above (view full size here).
The red and grey assemblies in the center of the image are “conveyor beds.” Conveyors are custom-built fixtures that transport units to each “station” on the assembly line. Integrators fabricate the frames of the conveyor beds from stock steel. Industrial motors and roller systems built into the conveyor do the job of getting the units to physically move down the line.
This is just one example of a conveyance system. Conveyance comes in all shapes and sizes, depending on what is being manufactured. The conveyor beds shown above likely weigh around a ton. At the other end of the spectrum, you have smaller components like circuit boards. Conveyor systems of this size weigh only a few pounds.
The production units themselves ride through the conveyor system on “pallets.” Like a conveyor bed, a pallet is custom-built to hold and carry each unit down the line. Pallets ride through the automation, passing from conveyor bed to conveyor bed.
Additionally, pallets are built to precise tolerances. When a unit arrives in a station, the robots will need to do their work in as close to the exact same physical location on the unit each time. For this reason, pallets must all be almost exactly the same, so that each unit will sit in a very specific position at each station.
Pallets hold each unit in place with carefully sized pins, or by other mechanical means. This helps to ensure proper positioning. In industrial automation, “repeatability” is the name of the game. Repeatability is the idea that, in automation, the same exact thing should happen in the same exact place every time.
Systems Integrator Roles
What is an automated systems integrator responsible for in regards to conveyance? Integrators will likely build, position, level, and program the conveyor systems. Systems integrators must take great care in many details to ensure that the conveyors will run smoothly. As examples, beds must be precisely assembled and leveled, and many parameters must be programmed into each motor and drive system.
You can see many robots in the image above. Each robot is ready to go in and work on the next vehicle.
How does each robot know where to go to do its job? Integrators must carefully plan, teach, and test motion for each robot. Also, each robot must be programmed to control and receive feedback from its End Of Arm Tool, or “EOAT”.
The robot is just a means of moving the EOAT to where it needs to be. What is an EOAT? The EOAT is what actually gets the work done in a robotic factory. There are many different types of End Of Arm Tools. Some examples include:
Material Handlers: move parts from one place to another
Machine Vision: records data, locates parts, or provides error-prevention
Joining: welders, riveters, or other equipment that fastens parts together
The robots above look like they’re carrying weld guns. Weld guns are used to “spot weld” the frame of the vehicle together. Welders work by touching the top and bottom of a part of the vehicle. Then, they pass a high current from one side of the gun to the other. The heat generated by this current flow welds the metal of the vehicle together.
For this reason, each weld gun needs a weld controller. Each weld controller has to be setup to output a certain amount of power. Further, this power setting has to be carefully tested to ensure that the gun generates the right amount of heat to form a good weld.
Safety, Sensors, Feedback, And Motion
Gates, fencing, light curtains, E-Stop buttons, and other safety devices exist throughout the automation equipment to protect the people that work in the area
Also, sensors, switches, operator buttons, and other input devices inform the PLC on the status and position of various equipment
Lights, buzzers, displays, valves, motors, actuators, and other output devices move parts and help humans understand what the equipment is doing
Imagine the variety of these components present on a large automation line. There may be several sensors on one machine. Further, each sensor may work differently and come from a different manufacturer. An automated systems integrator must be able to install and configure all of these many, many different industrial automation devices.
Programmable Logic Controllers (PLC’s)
A “PLC” (Programmable Logic Controller) is the brains of the operation. PLC’s accept inputs from the equipment and sensors. The PLC then performs processing, and sets outputs based on its programming. These outputs then control the actions that take place in the automation.
For example, an air supply line has an analog pressure sensor. Our pressure sensor sends a signal to the PLC that represents the pressure read at the sensor. The PLC has been programmed to interpret the signal sent from the sensor. In the PLC’s logic, the signal’s value is converted back to a pressure reading.
Then, integrators have programmed the PLC to check this pressure reading against a minimum pressure. If the pressure reading is lower than the minimum pressure, the PLC turns on a pump. In this case, the PLC sets outputs to the pump that command it to turn on. This is an example of how integrators might program a PLC to manage air pressure in a supply line.
PLC’s must be carefully programmed for each application. This programming must take into consideration concerns for safety, quality, efficiency, ease of use, and repair.
Human-Machine Interfaces (HMI’s)
To allow operators to interact with the machinery without having to know how to program a PLC, there needs to be one or more “HMI” (Human-Machine Interface) panels. An HMI is a programmable display; it’s basically a fancy computer monitor. Using an HMI, someone can interact with the PLC through buttons and other input devices on the screen. Many modern HMI’s are rugged touchscreen interfaces, built to withstand the industrial environment.
What Jobs Are Available In Automated Systems Integration?
Often, a manufacturer will approach a systems integration company with a system it wants built. The integrator then designs and builds a complete automation solution that will assemble the manufacturer’s product. Building an automation line from scratch requires a variety of skills and talents.
Managers oversee the business side of the operation
Mechanical Engineers, Electrical Engineers, Automation Engineers, and engineers from other specialties will design the systems. Engineers will ensure that the equipment and programming meets the customer’s specifications and any appropriate codes and regulations. In this regard, engineers must dig into the little details to understand, for instance, what type of sensor will fit a particular application. Engineers may perform PLC programming, HMI design, and development of “templates” of logic for use in the PLC and robotics
Millwrights cut and weld large assemblies, operate lifting equipment, and fasten components to the building’s structure. Also, millwrights are often responsible for servicing and repairing large motors, gearboxes, and other heavy-duty mechanical devices
Toolmakers fabricate detailed components to tight tolerances
Similarly, PLC Technicians set up and program the controllers
Industrial Electricians wire and install a wide range of electrical components, and may also program PLC’s, robots, and other electronic controllers
These are the core positions that automation integration shops employ. That is, at least in terms of building the automated systems. There may also be any number of other administrative positions in marketing, sales, finance, and other fields. On the technical side of the house, integrators may also employ Software Engineers, IT Technicians, and Facilities Engineers.
What Is It Like Working For An Automated Systems Integrator?
A typical work flow for an integration project might be as follows:
Firstly, project planning and materials acquisition
Machine assembly and programming at the integration facility
Transportation to the customer
Once on-site, machine installation, debug, and trials at the customer facility
On-site support as the customer takes on ownership of the equipment
Lastly, project wrap-up
Work Life As An Automation Integrator
Given that many integration projects consist of building an automated assembly line from scratch, integration work often occurs at the customer location. Depending on the size of the integration shop and the size of the project to which you’re assigned, very high travel percentages may be required. In other words, automated systems integrators may spend as much as 90-100% of their time away from home.
Customers who have purchased large or complicated automation solutions may require support well in to the launch of the project. Because of this, on-site support requirements can range from days to years. If you are a competent member of the team or have done a lot of the programming on a certain line, the company may ask you to stay on the road for months. In this case, many companies allow for you to travel home every couple of weeks.
Providing support for the customer can be stressful. The automation equipment that your company has built is what the customer uses to make money. For this reason, they may not be very happy when it breaks down.
On the other hand, you may be able to land a design or commissioning position that does not require any travel. Those performing “machine assembly” at the shop may be able to enjoy a similar lifestyle to other 9-to-5 positions. Even so, many shops will have “surges” in the pace of work. There may be slow periods followed by stretches where your boss wants you working overtime every day of the week.
Pay And Benefits For Automation Integrators
The good news is that many integration shops offer very competitive wages and benefits. While on the road, many shops pay bothovertime and “Per Diem.” Per Diem is extra money the integration shop will pay you for each day away from home. This additional pay covers food and other costs.
There can be other perks of travelling. For example, opportunities to visit new places, work in cool facilities, and network with other professionals. When I have had to travel for automation work, I have generally been put up at nice or at least decent hotels and had dinner out on the company dime.
Whether or not you’re travelling, you may have the ability to participate in training and continuing education so that you can continue to grow technically. Not to mention, you’re building industrial automation equipment! If you’re a person who enjoys working with automation, it’s hard to find a more interesting or challenging job.
What Should I Study If I Want to Work For an Integrator?
Of course, the answer to this question depends on what type of position you’re pursuing.
For a position as a skilled tradesperson (millwright, toolmaker, electrician), see if you can land an internship
In certain areas, certificate or Associate’s programs may be available to help you get a job as a skilled tradesperson
For engineering, complete a Bachelor’s degree in the field of your choice (Electrical Engineering, Mechanical Engineering, etc.)
Automated systems integrators may also hire persons with degrees in Industrial Automation or Mechatronics
If you want to work as a PLC or robotics technician, you may be able to complete a local or online training program to help get your foot in the door
Industrial electricians with PLC or robot experience should be qualified for this type of position
Integrators make magic happen – they turn disparate systems into one large, cohesive “machine.” From custom assembly of heavy, metal fixtures, to robot and controller programming, an integration shop has to be able to do it all.
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When I was learning PLC programming, I remember scratching my head about some of the concepts surrounding proximity sensors. Digital or analog, Normally Open (NO or N.O.), or Normally Closed (NC or N.C.)? What exactly does it mean for a sensor to be NO or NC? What effect will it have when I’m checking the state of the sensor at the PLC or other controller?
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NO or NC refers to the way that a sensor is wired and in what state its output signal will be when the sensor is “made.” A sensor is “made” when an object is present that the sensor has been set up to detect. The characteristics of the sensor determine whether or not an object will detected. These characteristics can include its detection type (inductive, capacitive, ultrasonic, photoelectric, etc.), sensing range (how far away the part can be from the sensor), and other factors. The point of a proximity sensor, or “prox,” is to know that an object is there or not there. When a sensor detects an object, its output state changes.
Someone might refer to the types of proximity sensors described above as “digital proxes.” In this context, digital has a somewhat different denotation than the typical use of the word outside of industrial automation. If a sensor is “digital,” it only has two possible output states: on or off.
There are a multitude of different sensors on the market. There are small sensors, large sensors, laser sensors, sensors like the barrel proxes above which have no configuration whatsoever, sensors that require quite a bit of set up, and everything in between. If a sensor’s sole purpose is to detect whether or not an object is present somewhere, its output is typically digital (either on or off). For this reason, people sometimes refer to sensors of this type as “switches”. Like a light switch in your home, they either turn an output on or off.
In this regard, you can think of the behavior of a prox switch or other digital output as being just like that of the paperclip switch that turned on a small lightbulb in your 2nd grade science class. The prox sensor is the paperclip, and the target passing in front of the prox is your hand pushing down on the paperclip to change the switch’s output state.
Aside from digital outputs, there are devices with “analog” outputs. Analog sensors output a specific value within a range (anywhere from 2V to 10V, for instance). As one example, sensors with analog outputs can be used to tell a machine how far away something is.
As mentioned above, the purpose of a proximity sensor is to tell a machine when something is present in front of the sensor. So, what actually happens when the sensor detects an object? Well, the sensor’s output changes state. This means that the sensor either energizes an output signal wire with a small amount of electricity, or not.
Like a light switch at your house that is off, an NO sensor will not, by default, put out a voltage to its output wire. Returning to the paperclip circuit analogy, an NO sensor’s default state is similar to the paperclip lifted off the thumbtack. The switch breaks the output circuit by default; hence, the output circuit is “normally open”. Referring to the graph above, when an NO sensor is in its default state (does not detect a target), the sensor’s output is off.
What happens when the sensor is made?
When an appropriate object passes within the sensor’s sensing range, the sensor outputs a voltage through its signal wire. This signal can indicate to a controller that the target has “made” the sensor. So long as the target remains within sensing range, the prox will continue to provide voltage on its output signal.
What’s the point of this? This is how the sensor “tells” the controller: “hey, I’m energizing my output as a signal to you that there is something in front of me right now.”
As you can see in the graph above, once the object passes out of the range of the sensor, the sensor will turn off its output. A controller would now see that the sensor is in its normal, “off” state.
As a brief aside, there are quite a few ways to refer to something as being “on” or “off”. Below are some other ways you might hear someone refer to a signal as being on or off. In my opinion, all of these are more or less equivalent:
Normally Closed Devices
NC sensors and other devices behave exactly opposite to NO devices in regards to their outputs. NC devices are, as indicated by their name, normally closed, meaning that their output is onby default. Only when an object makes the sensor does the signal actually turn off. Here’s a simplified graph of the signal behavior for an NC sensor:
If you understood the behavior of Normally Open sensors, then you also understand the behavior of Normally Closed sensors; one is simply the inverse of the other. If an NO and NC sensor were set up to detect the same object, the NO sensor’s output would be on when the NC sensor was off, and vice-versa.
Default Output State
Output State When Sensor Is Made
Why choose an NO or an NC sensor?
Due to these differences in output behavior, Normally Open and Normally Closed sensors are better or worse for certain applications.
All cables and electrical components will eventually fail. To get an idea of why you might choose one sensor or another, let’s first talk about how we want our systems to behave when a cable or sensor is damaged, and we no longer get the signals we’re relying on to control machine motion.
The two most common types of electrical failures are “opens” and “shorts,” with opens being the most common. An open is an unwanted break in a circuit. Cuts, crushing, or other damage to the cable can cause an open.
An example of a Normally Closed application: Emergency Stop
Modern factories are populated throughout with “E-Stop buttons”. Emergency Stop buttons can be used by anyone in the facility if an unsafe condition is observed. Slap an E-Stop, and all machine motion will come to a halt as quickly as possible.
Remember that you can think of a prox sensor as just another type of switch. What we traditionally think of as a switch is usually switched by mechanical action. Proxes are typically solid-state devices with internal electronics that turn outputs on and off. An E-Stop is an example of a true mechanical switch. When someone presses an E-Stop, metal contacts inside of the device open or close its output circuits.
NC or NO?
Let’s consider whether the E-Stop should be a Normally Open or Normally Closed device. With a Normally Open E-Stop, the button’s outputs will be off (open) when the button is in its default (not pressed) state.
In an emergency, someone hits the E-Stop. The mechanical action of pressing the button causes the normally open contacts to close, energizing the button’s outputs. Now the controller can detect those outputs, and we can use this status in our logic to halt machine motion. Cool.
Except… let’s return to the concept of an unwanted break in our circuit. What happens if the cable that connects the E-Stop button to the controller has been damaged?
If the E-Stop is a Normally Open device, and its cable becomes damaged, then when we go to activate the E-Stop, we will never get a signal back to our controller telling it to halt production. To the controller, a damaged electrical system and the default output of a Normally Open switch look exactly the same. In either case, there would be no incoming voltage to the controller’s input.
If the E-Stop in this example were Normally Open, you would only check for its output signal when you needed it to stop the line. As a result, you have no way of knowing whether the button or cable is damaged until it’s too late. A Normally Open switch wouldn’t just be a bad choice for this application, it would be dangerous. In an emergency, an ineffective E-Stop could contribute to someone being severely injured or killed.
Making the right choice for the right application
For this reason, E-Stops and most safety devices are Normally Closed. When a Normally Closed E-Stop is in its default position, the contacts close the circuit and return a signal to the controller indicating that the system is safe. Because the E-Stop returns a signal constantly, any condition that causes the E-Stop signal to go low will be detected. Aside from someone actually pressing the button, some other possible causes for losing the E-Stop safe signal might include loss of power to the system, failure of the E-Stop’s cable, or failure of the E-Stop button itself.
Now, since our Normally Closed E-Stop is always sending a signal back to the controller when it’s in the safe position, we set our logic up so that we must constantly see the signal from the E-Stop to allow the factory to run. You could think of this type of Normally Closed signal as a constant “thumbs-up” to the controller that the system is safe. In the controller logic, machine motion would only be permitted when the expected signals from all safety devices are present.
Along this same line of thought, other sensors that detect unsafe conditions, such as tank overfill, are typically Normally Closed. Because NC sensors return a signal by default, any loss of that signal will immediately indicate that the system is not safe.
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An example of a Normally Open application: Part Present
For less safety-critical applications, Normally Open sensors work just fine and in fact are found more commonly in industrial automation than NC sensors. In certain cases, use of an NO sensor would actually be preferable, and many people find it easier to interpret the behavior of NO sensors when it comes time to debug an electrical or programming issue.
“Part Present” applications, for instance, often use NO sensors. Let’s say that you want a robot to pick up a part and move it to another location. When the robot moves to the “pick position,” you want to be able to verify that the part is positioned in the robot’s “end effector” before allowing the robot to attempt to move the part. An end effector is a fixture bolted to the robot arm that is custom-built for picking up a particular part.
Normally Open sensors are ideal for this type of Part Present detection, as they only send the signal that the part has been picked up if they actively sense material. If a cord or sensor is damaged in this type of application, the sensor will simply never output its signal. Because the robot won’t see the necessary signal, robot motion will halt until the problem can be corrected.
NC and NO Sensors
There’s a common thread in both the Normally Closed and the Normally Open applications described above. With either NO or NC, you want positive indication before you allow the system to move. By positive indication, I mean that you want the PLC to see the signal from the sensor go high.
In the E-Stop application, you want to be able to move the system by default. You only want to disable motion if a certain condition is met (someone slaps the E-Stop). Hence, you want the signal to be on by default (Normally Closed). You only want the signal to go low if your system isn’t safe.
In the Part Present application, you want the robot to stop at the pickup position by default. You only want to enable motion under certain conditions (the part positioned properly in the end effector). Hence, you want the signal to be off by default (Normally Open). You only want the signal to go high if your part is properly loaded.
Hopefully, this has shed a bit of light on some of the basics of proximity sensors, including the concepts of Normally Open and Normally Closed. There is a lot to be said about the many sensors on the market and their functionality. Click the following link for an in-depth look at the various types of sensors and how they work.
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